This project will examine variation in cardiac and respiratory patterning during different sleep and waking states in normal infants and infants at risk for the sudden infant death syndrome (SIDS). The objective is to determine the nature and timing of cardiac and respiratory rate and variability differences found during different sleep-waking states between normal infants, sibling of SIDS victims, and infants destined to die of SIDS. These studies will be carried out on 12-h polygraphic data already collected and observer state-classified from normal infants and siblings of SIDS victims over the first 6 months of life, as well as from a set of 24-h ambulatory recording from a set of normal infants and infants who subsequently died of SIDS. Minute-by minute classification of sleep states will be determined on the ambulatory data from machine assessment of cardiorespiratory parameters. Variation in cardiac beat-to-beat intervals and respiratory periods will be determined with time domain peak- trough assessment and with frequency domain techniques. Rate and variability comparisons, controlled for age and state, will be made across risk groups using multivariate techniques. Coupling of cardiac rate variation with moment-to-moment respiratory changes will be assessed with time series procedures. The rate and variability changes exhibited in risk groups will be partitioned by time of night, and by relationship to the 24-h and 3-4 rhythms.